1. Field of the Invention
The invention is related to high-speed gain control of Erbium-Doped optical Fiber Amplifiers (EDFA), in particular to a method and an apparatus applying self-adapting feed forward control, which can automatically correct the feed forward control parameters according to the variation of EDFA parameters due to environmental condition changes or device aging.
2. Background
In optical-fiber communication systems, the optical fiber amplifier, especially the erbium-doped fiber amplifier (EDFA), has become one of the key parts in optical communication systems. As an EDFA can provide simultaneous amplification for multiple wavelength channels, it greatly accelerates the development of wavelength division multiplexing (WDM) system at low cost over the last decade. However, in dynamically configured WDM systems, random add/drop of channels can take place, which tends to give serious transient effect on the surviving channels for WDM system using EDFAs.
The transient effect mainly results from an abrupt change in the number of channels in WDM system, which gives rise to the cross gain saturation effect on the surviving channels in an EDFA. When the number of channels decreases, the input power to the amplifier is reduced, leading to amplification gain spike of the surviving channels, which can result in bit error rate (BER) increase through optical non-linear effect or receiver overloading. On the other hand, when channels are added, the input power to the amplifier increases, the existing channels will experience a temporary gain decrease, which tends to reduce the signal-to-noise ratio, leading to the BER increase. In order to maintain the transmission performance of the WDM systems using EDFAs in the event of channel add/drop, it is necessary to keep the above transient effect under control. One way to do it is to implement high speed gain control in EDFAs so that the amplifier gain is kept at an approximately constant level with the number of channels being added or dropped.
An effective way to achieve high speed gain control is to use a combination of feed forward and a classic proportional-integral-derivative (PID) feedback control methods.
The feed forward control will adjust the pump power according to the input power change before the gain saturation process takes effect while the feedback control will make fine adjustment to achieve the gain convergence to the gain set point. In this arrangement, the feed forward control brings the amplifier quickly to the pump power set point that is very close to its steady-state target and the feedback PID control based on the error of the output power to its target is more accurate but relatively slow due to finite response time of the gain cross saturation effect. As a result, the degrees of gain over/under shot due to channel add/drop will be dependent on the accuracy of the feed forward control. To certain extend, the relation between the pump power and input signal power at a given gain target can be calibrated when the amplifiers are manufactured. However, this relation can change due to ambient temperature variation or device aging over its life time.
The slope efficiency and threshold of pump laser and the insertion loss of passive components in an EDFA, can change over temperature and over its life time. This is especially true for the un-cooled pump lasers which are sometimes favored due to low power consumption and compact size. Both slope efficiency and threshold of the un-cooled pumps are quite sensitive to operating temperature and can change significantly over time. In general, an EDFA is a time-varying system whose parameters change with time. In general, the calibrated relation between pump and input signal power will become inaccurate over time, which will result in the degraded transient control performance of the amplifier. To overcome this drawback, a self-adaptive feed forward control method can be used to dynamically correct the feed forward accuracy.